Various padding devices have been employed in the past. Examples include liquid- or gas-filled bladders, e.g. water-filled cushions and pneumatic pads; and gases or liquids dispersed in a solid material, e.g. foams and gels. Generally, such padding devices operate on the principle of conformation to the shape of an object when placed under pressure. When a force, such as a person's mass, is placed on such a padding device, the device deforms so as to conform to the shape of the pressure applying object in order to distribute the force over as large an area as possible. These devices perform adequately when the object being padded has a relatively large, uniformly shaped surface area. However, when the object being padded includes a relatively small area of concentrated force, such as that caused by a protuberance, the majority of known padding devices do not perform to adequately reduce the discomfort of users in many applications. This is because such padding devices exert greater responsive pressure on the area of concentrated force.
The reason for the greater pressure is that materials employed in prior art padding devices typically have a high degree of "memory." As used herein, the term "memory" will refer to that characteristic of a material in which the material returns to its original shape as a result of internal restoring forces when an external force is removed. Such materials deform to the shape of an object which applies an external force by compressing. However, due to the internal restoring forces, a pressure which is proportional to the degree of compression is exerted against the object which applies the external force. A sharp protuberance compresses the padding device more than the surrounding areas and, as a result, the padding device presses back with greater pressure in these areas of high compression. Such areas of high pressure are especially undesirable when the protuberance is a bone, such as an ankle or ischial tuberosity. The high pressure can lead to discomfort and, after periods of extended use, to actual damage to the tissue overlying the protruding bone.
The problem can be described with reference to a padding device comprising a gas dispersed in a solid material, e.g. foam. Tiny gas bubbles in foam act like millions of coil "springs." When required to conform to an irregular shape, such as a human body, the "springs" are compressed to varying degrees, each pushing back on the body with a force proportional to the amount of compression. Intimate conformity is best obtained with a relatively soft foam, which can be compared to weak "springs." The pressure on protuberances, where the "springs" are greatly compressed, will be relatively high, possibly causing pain and reduced circulation. The problem is even more pronounced if a stiffer foam is employed, because the "springs" are stronger.
Deformable silicone gel padding devices are disclosed in U.S. Pat. No. 3,449,844 by Spence, issued June 17, 1969; U.S. Pat. No. 4,380,569 by Shaw, issued Apr. 19, 1983; U.S. Pat. No. 3,663,973 by Spence, issued May 23, 1972; U.S. Pat. No. 3,548,420 by Spence, issued Dec. 22, 1970; U.S. Pat. No. 3,308,491 by Spence, issued Mar. 14, 1967; U.S. Pat. No. 4,019,209 by Spence issued Apr. 26, 1977; and U.S. Pat. No. 4,668,564 by Orchard, issued May 26, 1987. In U.S. Pat. No. 4,380,569, a silicone gel containing glass microbeads is disclosed.
The silicone gel disclosed in these patents is described as having near total memory. In other words, it returns to its original shape when an external force is removed. The internal restoring forces necessary to provide such memory are undesirable in some applications. In use, differential pressures will result depending upon the degree of deformation of the silicone gel material, with higher deformation resulting in localized areas of high pressure being exerted on the external pressure applying object.
In order to alleviate the problem of differential pressure inherent with many prior art materials, flowable, pressure-compensating materials were developed. Such materials and applications thereof are described in U.S. Pat. No. 3,402,411 by Alden Hanson, issued Sept. 24, 1968; U.S. Pat. No. 3,635,849 by Alden Hanson, issued Jan. 18, 1972; U.S. Pat. No. 4,038,762 by Swan, Jr., issued Aug. 2, 1977; U.S. Pat. No. 4,083,127 by Chris Hanson, issued Apr. 11, 1978; U.S. Pat. No. 4,108,928 by Swan, Jr., issued Aug. 22, 1978; U.S. Pat. No. 4,144,658 by Swan, Jr., issued Mar. 20, 1979; U.S. Pat. No. 4,229,546 by Swan, Jr., issued Oct. 21, 1980; and U.S. Pat. No. 4,243,754 by Swan, Jr., issued Jan. 6, 1981. Each of these U.S. patents is incorporated herein by reference in its entirety. These patents will collectively be referred to as the "flowable, pressure-compensating material patents."
The preferred materials disclosed in U.S. Pat. No. 3,402,411 comprise from 20 to 25 weight percent polyisobutylene, from 25 to 37.5 weight percent of an inert oil, e.g. mineral oil or a saturated ester oil or a mixture thereof and from 42.5 to 50 weight percent inorganic filler. U.S. Pat. No. 3,635,849 discloses a composition consisting essentially of from about 5 to about 45 weight percent of a polyolefin, particularly polyisobutylene, from about 15 to about 70 weight percent of a paraffin and from about 5 to about 80 weight percent oil. Lightweight aggregate materials, for example, polystyrene beads or a heavy aggregate such as Fe.sub.3 O.sub.4 can also be added.
The flowable, pressure-compensating materials disclosed in U.S. Pat. Nos. 4,038,762, 4,108,928 and 4,243,754 include from 21.39 to 77.96 weight percent oil, 21.04 to 69.62 weight percent wa&gt;: and 1 to 9 weight percent microbeads. These patents teach away from the use of water in the finished product stating that "Since water generally increases the specific gravity of the finished fitting material, and does not serve any functional or necessary purpose, as such, in the finished fitting material, it is very desirable that if it is present in the finished fitting material, that it not be present in amounts or levels that exceed tolerable, minimal or residual levels (e.g. up to or not exceeding about 8% by weight, preferably up to not exceeding about 3% or about 5% by weight)."
U.S. Pat. Nos. 4,144,658 and 4,229,546 disclose flowable, pressure-compensating materials comprising 10 to 60 weight percent hollow, glass microbeads, 8.5 to 34 weight percent wax and 26.5 to 81 weight percent oil. U.S. Pat. No. 4,083,127 discloses a flowable, pressure-compensating fitting material consisting essentially of discrete, lightweight, sturdy microbeads distributed throughout a continuous phase of wax and oil.
In use, the flowable, pressure-compensating materials disclosed in the above-mentioned patents are typically placed in a pliable package, such as between two leak-proof resinous sheets which are sealed at the edges. The flowable materials act hydraulically. An applied force causes flowable material to migrate from areas of higher pressure to areas of lower pressure until pressure throughout the package is uniform. Once conformity has been achieved, force is distributed substantially equally over the entire surface of the package thus alleviating the differential pressure problems associated with prior devices. The viscosity of the flowable materials can be varied. Higher viscosity does not decrease the ability of the flowable materials to conform to the shape of the pressure applying object, only the rate at which they will migrate to conform. Flowable materials are presently marketed under the trademark FLOLITE.TM. by Alden Laboratories, Inc. of Boulder, Colo. U.S.A.
FLOLITE.TM. brand materials have performed exceptionally well in a number of applications, and have gained wide commercial acceptance in the marketplace. In spite of this commercial success, it would be advantageous to provide novel compositions which are useful as flowable, pressure-compensating materials. For example, it would be advantageous to provide a composition which exhibits a higher degree of flame retardancy than present flowable, pressure-compensating materials. It would be advantageous if the number of components required to provide a flowable, pressure-compensating composition were reduced to a minimum. In this way, it would be possible to simplify manufacturing. It would be advantageous if the materials used in a composition were relatively inexpensive in order to reduce raw material costs. It would be advantageous if the composition were to be less prone to separation than currently-employed materials. It would be advantageous if spherical particles included in a flowable, pressure-compensating composition would not "float out" of the composition. It would be advantageous if the viscosity of the composition was relatively stable over broad temperature ranges. It would be advantageous if the viscosity of the material could be controlled in such a way so as to ease manufacture of devices containing the composition.